Seasonal migration changes

Changes in fall and spring centers of biomass

To narrow down the potential list of species interactions, I’m pulling out a number of species whose distance between their fall and spring centers of biomass have changed significantly (p value <= 0.05). This has narrowed our list of 29 predatory species down to 8. Some have increased their distance between centers and some have decreased. We will classify these species with their movement classifications, examine how prey species are moving, and assess whether those patterns are consistent with the previously hypothesized overlap patterns.

Movement and overlap

Previously, we described the movement patterns of roughly 40 species caught in the NEFSC Bottom Trawl survey. The movement patterns of the 8 species exhibiting the strongest changes in seasonal center of biomass, as well as 5 pre-determined prey species (silver hake is both), are described below. Prey species are noted in italics.

  • Contract: Goosefish

  • Lean: Atlantic herring, Longfin squid, Northern sand lance, Scup, Thorny skate, Winter skate

  • March: Alewife, Red hake, Silver hake, Smooth dogfish

  • Expand: Spiny dogfish

In our overlap analysis, we’ve described seven of the eight above species as predators, with silver hake representing prey species, along with alewife, Atlantic herring, longfin squid, and northern sand lance. We compare the trends of species distributions, then describe the expected relationship based on our movement matrix. We will then map the spatial intersection at the decadal time scales to see if hypothesized overlap pattern is consistent with the trawl survey results. It is likely that we will include other environmental variables, such as average depth, sea surface and bottom temperature.

Pred/Prey Stable March Expand Contract Lean Retract
Stable = / 0 + / - + / = - / = + / - / 0 + / - / 0
March + / - = / 0 + / = + / - + / - + / -
Expand + / = + / = = / 0 + / = + / = - / 0
Contract - / = + / - + / = = / 0 + / - - / =
Lean + / - / 0 + / - + / = + / - = / 0 - / 0
Retract + / - / 0 + / - - / 0 - / = - / 0 = / 0

The most frequent predator-prey overlap type we see is lean-march, which depending on the initial conditions could results in an increase or decrease of overlap.

Predator Prey Predator movement Prey movement
Goosefish Alewife Contract March
Atlantic herring Contract Lean
Longfin squid Contract Lean
Northern sand lance Contract Lean
Silver hake Contract March
Red hake Alewife March March
Atlantic herring March Lean
Longfin squid March Lean
Northern sand lance March Lean
Silver hake March March
Scup Alewife Lean March
Atlantic herring Lean Lean
Longfin squid Lean Lean
Northern sand lance Lean Lean
Silver hake Lean March
Smooth dogfish Alewife March March
Atlantic herring March Lean
Longfin squid March Lean
Northern sand lance March Lean
Silver hake March March
Spiny dogfish Alewife Expand March
Atlantic herring Expand Lean
Longfin squid Expand Lean
Northern sand lance Expand Lean
Silver hake Expand March
Thorny skate Alewife Lean March
Atlantic herring Lean Lean
Longfin squid Lean Lean
Northern sand lance Lean Lean
Silver hake Lean March
Winter skate Alewife Lean March
Atlantic herring Lean Lean
Longfin squid Lean Lean
Northern sand lance Lean Lean
Silver hake Lean March

Scup & Herring Example

The map below shows the annual intersections in decadal snapshots. We see that Scup and Atlantic herring both exhibit leaning distribution patterns and appear to be increasing in overlap. It’s important to note that these intersection points are not centers of biomass but survey points in which both species were caught. We will need to run a regression analysis to determine if this increase in overlap is statistically significant, supporting our previous hypothesis that predators and prey species exhibiting the same distribution patterns will have an increase in overlap.

Annual center of biomass

Survey catch

It will likely be necessary to plot the quantiles (5th, 50th, and 95th) to properly visualize the overlap over time.